1. Field of the Invention
The present invention relates to a solid-state image pickup device represented by a complementary metal-oxide semiconductor (CMOS) image sensor, and to a camera system.
2. Description of the Related Art
In recent years, CMOS image sensors have been attracting attention as solid-state image pickup devices (images sensors), in place of charge-coupled devices (CCDs).
This is because CMOS image sensors overcome the following problems.
That is, the problems include that a dedicated manufacturing process is necessary for fabricating CCD pixels, a plurality of power supply voltages are necessary for the operation thereof, and it is necessary to cause a plurality of peripheral integrated circuits (ICs) to be operated in a combined manner.
CMOS image sensors overcome these various problems of CCDs, such as that the system becomes very complicated.
CMOS image sensors can be manufactured using a manufacturing process similar to the process of manufacturing general CMOS ICs. Also, a CMOS image sensor can be driven by a single power supply. Furthermore, an analog circuit and a logic circuit using CMOS processes can be mixed in a single chip.
Accordingly, the number of peripheral ICs in a CMOS image sensor can be reduced. That is, CMOS sensors have multiple great advantages.
An output circuit of a CCD is generally a 1-channel (ch) output using a floating diffusion (FD) amplifier with a floating diffusion layer.
In contrast, a CMOS image sensor has an FD amplifier in each pixel and generally uses a column-parallel output scheme that selects a row from an array of pixels and simultaneously reads and outputs signals from the row in a column direction.
Because it is difficult to obtain sufficient drive power using the FD amplifiers arranged in the pixels, the data rate is necessary to be dropped. In this sense, parallel processing is regarded to be advantageous.
Such CMOS image sensors have been widely used as image pickup devices in image capturing apparatuses such as digital cameras, camcorders, monitoring cameras, and in-vehicle cameras.
The technique of adding output signals from multiple photodiodes (PDs) with different sensitivities and outputting the sum signal as an output signal from a pixel is effective as a method of realizing a CMOS image sensor with a high dynamic range. In particular, buried photodiodes (BPDs) are widely used as PDs. Since there is a surface level due to defects such as dangling bonds on the surface of a substrate on which PDs are formed, a great amount of electric charge (dark current) is generated owing to the thermal energy. As a result, it becomes difficult to read a correct signal. In the case of BPDs, electric charge accumulating portions of PDs are buried in the substrate. In this way, the amount of dark current introduced into the signal is reduced.
The sensitivity of a PD can be changed by changing the exposure time or by providing a neutral density (ND) filter.
This method has the following advantages:
A higher dynamic range than that achieved by simply using a large pixel can be achieved; and
Although the output relative to the amount of incident light is nonlinear, the output can be easily changed back to be linear. When a color image is obtained, it is easy to perform color processing.